1. Field of the Invention
The present invention relates to a process for a continuous removal of nitrates from nitrate containing solutions by extracting the solution in question with an organic amine which is in a salt form substantially insoluble therein and by stripping the organic phase used for the extraction with a stripping reagent which, in addition to stripping, converts the amine, for the next extraction cycle, into the same salt as is used in the extraction, whereby the concentration of the salt formed by the cation of the stripping salt and the nitrate anion in the stripping solution is kept so high that this salt is crystallized from the stripping solution substantially continuously, but still so low that the nitrate anion extracted into the organic extraction solvent phase continuously passes into the stripping solution during the stripping while the anion of the stripping salt passes from the stripping solution into the organic extraction solvent phase, and the crystallized nitrate salt is recovered from the stripping solution.
2. Description of the Prior Art
It has been previously known to use for the extraction of nitrates a primary, secondary or tertiary aliphatic and/or aromatic amine which is substantially water insoluble or which can be made water insoluble. It is preferable to use a secondary amine with the formula R'--NH--R", wherein R' and R" are branched hydrocarbon chains containing a total of approx. 24-27 carbon atoms. One example of such an amine is the secondary amine available under the trademark AMBERLITE LA-1 (molecular weight approx. 351-393, manufacturer Rohm & Haas, Philadelphia, U.S.A.), which is known to be applicable to the extraction of nitrates and several metal ions, among others. To achieve the requisite settling and to maintain suitable extraction conditions in the extraction apparatus, the above secondary amine is dissolved in some suitable extraction reagent diluent to form a solution of the suitable concentration.
It is known that when the said amine is brought into a suitable salt form, which is thereafter contacted with the nitrate-containing solution to be extracted, the nitrate anion changes place with the anion of the amine salt while the nitrate anion passes into the organic phase and the anion of the said amine salt into the aqueous phase.
It is also known that the above nitrate salt of the amine can be contacted with another aqueous phase, one that contains a stripping reagent which reconverts the amine into the same salt that was used for the extraction, whereby the nitrate anion respectively passes from the organic phase into the said second aqueous solution.
This known liquid anion exchange process can be illustrated with the following simple balance reaction equation: EQU (R'R"NH.sub.2 A).sub.org + NO.sub.3aq .sup.-.revreaction. (R'R"NH.sub.2 NO.sub.3).sub.org + A.sub.aq.sup.- (I)
wherein A.sup.- stands for some anion. The extraction takes place in a known manner in the direction of the upper arrow of the balance equation and the re-extraction, or stripping, in the direction of the lower one. It is known that the reaction can be caused to occur in the direction of the lower arrow when the concentration of the A.sup.- salt in the stripping solution is kept high enough.
The present invention especially relates to a method of the type specified in the introduction.
Such a method is known from Japanese Publication No. 48-14318/73, according to which the stripping is carried out with a very acid salt solution (4-5 mol H.sup.+/kg), for which reason a temperature of 0.degree.C, preferably -20.degree.C, must be used for crystallizing KNO.sub.3 from the said stripping solution, since the solubility of salts increases when the acidity of the salt solution increases. Owing to the low temperature the industrial application of such a method is extremely costly and complicated. In addition, the organic phase (especially heavy amine) is highly viscous at such low temperatures and therefore poorly flowing.
Furthermore, the extraction of nitrates and the production of KNO.sub.3 are carried out from a strong nitric acid solution, i.e. from a very acid solution pH 0.